Abstract: Evolution of rock joints into transfixion is described by the time series from strain-stress curves,and the state of joint system in sandstone is chaotic before its ultimate state.Based on rock mechanical tests and evolution of time series,quantitative criteria to judge whether joints in sandstone specimens had been transfixed were established by Kolmogolov entropy to determine K.These criteria were used to study the ultimate strength of rock,and some relations were set up between macro mechanical parameters and Kolmogolov entropy which was a micro mechanical parameter.